A conventional variable cycle gas turbine engine includes a core engine driving a fan, and a bypass duct surrounding the core engine which is in flow communication with the fan. A conventional bypass valve is disposed at an upstream, inlet end of the bypass duct and is positionable in a closed position which substantially blocks flow from the fan into the bypass duct under certain conditions in the flight envelope of an aircraft being powered by the engine while allowing flow from the fan to be channeled into the core engine. The bypass valve is also positionable in an open position which allows substantially unobstructed flow from the fan into the bypass duct for bypassing a portion of the fan air around the core engine while allowing the remaining portion of the fan air to be channeled through the core engine during operation of the aircraft at other conditions in the flight envelope.
Conventional bypass door assemblies are relatively complex and are controlled in accordance with predetermined schedules corresponding to operation in the flight envelope of the aircraft. An exemplary conventional bypass valve assembly includes an annular ring valve which is translatable to open and close an annular inlet to the bypass duct. Conventional linkages and servovalves are used to translate the valve and are operatively connected to the control system of the engine for being responsive to the predetermined schedules contained in the control system for opening and closing the bypass valve at various conditions in the flight envelope.
In the open position, the bypass valve must provide for substantially unobstructed flow into the bypass duct for reducing or minimizing pressure losses therefrom which would decrease performance of the engine and reduce the cooling ability of the bypass air channeled in the bypass duct. The bypass air is typically used to cool downstream structures in the engine, such as, for example, a conventional augmenter and variable area exhaust nozzle, and any pressure losses due to the bypass duct would have to be accommodated, typically by increasing pressure in the bypass duct which decreases engine performance. Furthermore, the bypass valve must also provide for substantially unobstructed flow into the bypass duct to prevent or minimize any back pressure on the fan which could undesirably reduce stall margin of the fan. This factor is even more significant in an engine wherein the fan stall margin has a "pinch point", or minimum value, which occurs at about the same condition in the flight envelope when the bypass valve must open fully.
The bypass valve in the form of a mode selector valve in a bypass engine is typically positioned either in a fully open, or a fully closed position, although intermediate positions may be desirable in certain other embodiments. Accordingly, the bypass valve is typically open or closed relatively quickly when required for operation in the flight envelope when transitioning between low and high bypass operation, for example.